1. Background of the Invention
The present invention generally relates to image processing devices such as digital copying machines and facsimile machines, and more particularly to an image processing device having a function of detecting a dotted-image area.
2. Description of the Prior Art
An image processing device having the function of detecting a dotted-image area is proposed in, for example, Japanese Laid-Open Patent Application No. 3-276966. In the above image processing device, information indicating a ratio of a dotted-image area to a predetermined local area of an image indicated by digital multivalue data is referred to in order to detect polar pixels located at upward and downward peaks (local maximum and local minimum) at which the ratio of the dotted-image area to the local area is changed. Then, it is determined whether predetermined pixels within a small area form a dotted-image candidate area or a non-dotted-image candidate area on the basis of an existence state of the polar pixels located at the above upward and downward peaks. Thereafter, a dotted-image area is detected from the dotted-image area candidate area.
However, the above image processing device has the following disadvantages. First, a hardware configuration for realizing the above dotted-image area detecting function is very complex and expensive because a circuit logic for detecting the polar pixels located at the upward and downward peaks at which the ratio in the local area is changed must be formed by a hardware configuration. Second, the upward and downward peaks cannot be detected precisely. Particularly, when the ratio of the dotted-image area to the local area is equal to a certain level, the upward and downward peaks cannot correctly be detected.